1. Field of the Invention
The present invention relates to optical pickup devices, and more particularly, an optical pickup device which includes a hologram element as a detection optical element and which carries out recording/playing of information in relation to an optical disc optically.
2. Description of the Related Art
In such an optical pickup device that carries out recording/playing of information in relation to an optical disc optically, there have been adopted an astigmatism method for detecting a focus-servo error signal and a push-pull method for detecting a tracking-servo error signal.
FIG. 1 is a structural view showing one example of a conventional optical pickup device. In the figure, light radiated from a light source 1 is reflected by a beam splitter 5, transmitted through a collimator lens 4 and converged by an objective lens 3 to form a light spot 2a in an optical disc 2. Then, the light (the light spot 2a) is reflected by the optical disc 2 and enters the objective lens 3 again. Subsequently, the light spot 2a is transmitted through the collimator lens 4, the beam splitter 5 and a cylinder lens 6, in this order. Astigmatism is imparted to the light since it passes through the cylinder lens 6. Then, the light enters a photo detective element 7 having four divided detective areas placed in two rows and two columns, forming a light spot on the photo detective element 7.
As the light entering the photo detective element 7 has astigmatism imparted by the cylinder lens 6, if changing a relative distance between the objective lens 3 and the optical disc 2, then the light spot formed in the photo detective element 7 changes its configuration as shown in FIGS. 2A, 2B and 2C. That is, by picking up outputs A, B, C and D of four-divided detective parts 7A, 7B, 7C and 7D and successively calculating the expression (A+C)−(B+D) from these outputs, it becomes possible to detect a focus error signal in accordance with the astigmatism method.
Regarding detection of a tracking error signal, there is utilized a phenomenon that if a relative position of the light spot 2a to a track in the optical disc 2 in its radial direction changes, an optical-power distribution (in a radial direction) of the light spot formed on the photo detective element 7 changes as shown in FIGS. 3A, 3B and 3C with respective parts 7d, 7e and 7f. That is, by picking up the outputs A, B, C and D of the four-divided detective parts 7A, 7B, 7C and 7D and successively calculating the expression (A+B)−(C+D) from these outputs, it becomes possible to detect a tracking error signal in accordance with the push-pull method.
As a reference, FIG. 3B shows an optical-power distribution when there is no deviation in tracking (misalignment), while FIGS. 3A and 3C show optical-power distributions in case of misalignments in different directions. Throughout FIGS. 3A to 3C, shaded areas designate parts bedimmed as a result of interference between ±first diffraction lights and zero-order diffraction light in the reflection light from the optical disc.
According to a differential phase detection method as a standard method for detecting a tracking error in DVD (Digital Versatile Disc), it is also possible to detect a tracking error signal by comparing a phase of a calculation signal of (A+C) with a phase of a calculation signal of (B+D). Additionally, information recorded on the optical disc 2 can be detected by calculating the expression (A+B+C+D) from the outputs A, B, C and D of the four-divided detective parts 7A, 7B, 7C and 7D of the photo detective element 7.
As mentioned above, since the calculating of the outputs A, B, C and D of the four-divided detective parts 7A, 7B, 7C and 7D allows two kinds of servo-error signals and the signals recorded in the optical disc 2 to be detected, all of the astigmatism method, the push-pull method and the above-mentioned “phase-difference” method have advantages of easiness in calculation and small number of output terminals. This is one reason why these methods have been used widely.
However, if a misalignment is produced in the relative positions of the beam splitter 5, the collimator lens 4, the objective lens 3, the cylinder lens 6 and the photo detective element 7, then the position of the light spot formed on the photo detective element 7 would change as shown in FIGS. 4A and 4B. In such a case, it is impossible to detect an error signal precisely due to variations in the outputs of the four-divided detective parts 7A, 7B, 7C and 7D of the photo detective element 7. Therefore, the conventional pickup device of FIG. 1 is required to establish the relative positions of these constituents with high accuracy.
Further, since the respective components (i.e. the light source 1, the beam splitter 5, the collimator lens 4, the objective lens 3, the cylinder lens 6 and the photo detective element 7) have to be disposed apart from each other by predetermined distances, the arrangement might be an obstacle in miniaturizing the photo pickup device. As it is unavoidable that the performances of the cylinder lens 6, the beam splitter 5 and the collimator lens 4 and the assembling positions of constituents of the light source 1 (including a semiconductor laser) fluctuate, it is required to adjust the assembling positions of the cylinder lens 6 and the photo detective element 7, taking a lot of trouble and time with the production of an optical pickup device. Further, depending on variations per hour in the assembling positions of the constituents, it becomes impossible to detect a designated error signal.
As another focus-error detecting method for remedying these shortcomings of the astigmatism method, there is a spot-size method adopting a hologram element, in practical use (e.g. see Japanese Patent Laid-open Publication No. H05(1993)-101417). FIG. 5 shows one example of a conventional optical pickup device using such a hologram element. In operation, light radiated from a light source 8 is transmitted to an objective lens 12 through a hologram element 10 and a collimator lens 11 successively. By the objective lens 12, the transmitted light is converged to form a light spot 13a in the optical disc 13. Then, the light spot 13a is reflected by the optical disc 13 and subsequently transmitted through the objective lens 12 and the collimator lens 11, entering the hologram element 10.
Then, the incident light is diffracted into two lights by the hologram element 10 and emitted in the form of a first diffraction light 14a having a shortened focal length due to a convex-lens action of the element 10 and a second diffraction light 14b having an extended focal length due to a concave-lens action of the element 10. These diffraction lights 14a and 14b reach a photo detective element 9, forming respective light spots 9a and 9b thereon. Note that, as shown in FIGS. 6A˜6C, the photo detective element 9 comprises a first tripartition detective part 9A for the light spot 9a and a second tripartition detective part 9B for the light spot 9b. These tripartition detective parts 9A and 9B are connected with each other as illustrated in the figures, generating output signals F1, F2. Each of the tripartition detective parts 9A and 9B is composed of three lines and one column of detective components.
As the focal lengths of two diffraction lights 14a, 14b for the photo detective element 9 are different from each other, the light spots formed on respective detective surfaces of the tripartition detective parts 9A and 9B forming the photo detective element 9 change their sizes as shown in FIGS. 6A, 6B and 6C, corresponding to a change (increasing or decreasing) in the relative distance between the objective lens 12 and the optical disc 13. Thus, with the so-illustrated connection between the tripartition detective parts 9A and 9B and the calculation of the expression (F1−F2) using signals F1 and F2 outputted from the tripartition detective parts 9A and 9B, it is possible to detect a focus error signal in accordance with the spot-size method. In connection, information recorded in the optical disc 13 can be detected by calculating the expression (F1+F2) from the signals F1 and F2.
In the spot-size method for detecting the focus error signal, it is general that the photo detective element 9 is divided in only one direction (see FIGS. 6A˜6C) and the direction of respective parting lines coincides with a direction that would be obtained by projecting a radial direction of the photo disc 13 on the photo detective element 9. The reason for coincidence in direction is as follows. That is, when the objective lens 12 moves in the radial direction of the photo disc 13 in order to allow the light spot 13a to follow a track on the optical disc 13, the light spots 9a and 9b on the tripartition detective parts 9A and 9B move in the radial direction of the photo disc 13 as well. Accordingly, it is required to minimize the effects of light spots' moving on the photo detective element 9 on the focus error signal. This is the reason why the direction of parting lines coincides with the radial direction of the photo disc 13. In this view, it should be said that the above-mentioned push-pull method is not available since it is constructed to detect a tracking error signal by making use of a difference in the optical power distribution between left and right detective parts, which is similar to the operation of the photo detective element 7 in the astigmatism method.
Instead, it is often the case that the conventional photo pickup device using a hologram element is combined with a three-beam method (e.g. see Japanese Patent Laid-open Publication No. H11(1999)-283274). FIG. 7 shows this three-beam method schematically. According to the three-beam method, three light spots 15a, 15b and 15c are formed on a photo disc having pits (marks) 16, while a photo detective element (not shown) detects respective reflection lights of these light spots 15a, 15b and 15c. With the application of the three-beam method, a reflection light of the center light spot 15a formed on the photo disc is diffracted by the hologram element 10, so that one diffraction light enters the photo detective element 17A having the tripartition detective parts of FIG. 8 (corres. to the part 9A of FIGS. 6A˜6B), while the other diffraction light enters the photo detective element 17D having the tripartition detective parts of FIG. 8 (corres. to the part 9B of FIGS. 6A˜6B). In accordance with the afore-mentioned spot-size method, these diffraction lights are utilized to detect the focus error signals and the information recorded in the photo disc.
Further, the reflection lights of the light spots 15b, 15c arranged in the vicinity of the light spot 15a of FIG. 7 in the track scanning direction are respectively diffracted by the hologram element 10, so that two diffraction lights for the light spot 15b enter the photo detective elements 17B, 17E of FIG. 8, respectively and two diffraction lights for the light spot 15c enter the photo detective elements 17C, 17F of FIG. 8, respectively. Through the use of an action that the detective powers of the photo detective elements 17B, 17C and the photo detective elements 17E, 17F increase and decrease mutually in response to a change in the relative position of the light spot 15a to a track in the radial direction of the photo disc, the tracking error signal can be detected by calculating (T1−T2) from T1 as an addition output signal of the photo detective elements 17B and 17E and T2 as an addition output signal of the photo detective elements 17C and 17F.
The above-mentioned photo pickup device using the hologram element can allow a single hologram element 10 to cater for two actions of the beam splitter 5 and the cylinder lens 6 used in the conventional photo pickup device of FIG. 1. Additionally, as the hologram element 10 can be manufactured in small size and with high accuracy, it is possible to assemble the photo detective element 9 and the semiconductor laser (i.e. the light source 8) in the vicinity of the hologram element 10 integrally, facilitating a miniaturization of the optical pickup device in comparison with the conventional photo pickup device using the astigmatism method (FIG. 1).
Additionally, since the photo detective element 9, the semiconductor laser (the light source 8) and the hologram element 10 can be assembled to each other closely, it is possible to minimize a deviation of the relative position between the semiconductor laser and the hologram element 10 remarkably. Additionally, it is noted that the hologram element 10 is characterized in that a relative angle between an incident light into the element 10, which has been radiated from the semiconductor laser (the light source 8), and an exit light from the element 10, which has been reflected by the photo disc 13 and further diffracted by the element 10, becomes constant usually. Therefore, even if the relative positions of the hologram element 10, the collimator lens 11 and the objective lens 12 are subjected to some deviations, it is possible to form a light spot in a designated position on the photo detective element 9 so long as no deviation is produced in the relative position between the photo detective element 9 and the semiconductor laser (the light source 8).
Furthermore, even if a deviation is produced in the relative position between the photo detective element 9 and the light source 8, the focus error signal would not change so long as a direction of the parting line between the divided detective parts 9A and 9B of the photo detective element 9 is substantially identical to a direction of the deviation (due to no change in the optical power distributions in the divided detective parts 9A and 9B). The optical pickup device using the hologram element is characterized by the above-mentioned features and has various advantages (e.g. easiness for miniaturization, easiness for ensuring high reliability, etc.) in comparison with the conventional pickup device of FIG. 1.
In a photo pickup device in accordance with the spot-size method adopting a conventional hologram element, however, a method for calculating output signals is different from that in the above-mentioned astigmatism method. It means that it is impossible to convert electric signal processing circuits widely used in the astigmatism method to a circuits for the photo pickup device. Thus, the existing circuit for the astigmatism method has to be modified so as to meet with the spot-size method.
Alternatively, if adopting the push-pull method or the phase-difference method in order to detect the tracking error signal, it would be required to detect the optical power distribution of light spots on the photo detective element while being divided in two parts in the radial direction or in the track direction of the photo disc. Then, this requirement is accompanied with various modifications, for instance, dividing of the hologram element 10 into one area corresponding to an outer circumferential part of the photo disc and another area corresponding to the inner circumferential part, dividing of respective intermediate detective parts of the tripartition detective parts 9A and 9B (FIGS. 6A˜6C) in two furthermore.
Thus, although the conventional photo pickup device adopting a hologram element is easy to be small-sized in comparison with the conventional photo pickup device shown in FIG. 1, there is a limit to the miniaturization due to the large-sized photo detective element 9 and an increase in the number of output terminals. Further, since the calculation of output signals is complicated, drastic changes would be required for widely-used electric signal processing circuits dealing with the astigmatism method.
In this way, the conventional photo pickup device has been required to accomplish both the merits in the astigmatism method (i.e. easiness in calculation, reduced number of output terminals, capability of detecting the tracking error signal by the push-pull method or the phase-difference method without adding the photo detective element or increasing the number of output terminals) and the merits in the spot-size method using the hologram element (i.e. high reliability, easiness for miniaturization).